Go into settings > Wireless & network > Mobile Networks > Data Traffic.
After selecting / de-selecting that, the option will then appear in the drag-down bar.
The same way you'd do it on 1.6.
But that's not disabling 3G, that's just stopping apps from using your credit. To truly disable 3G, you'd have to change network mode to something like GSM, or 2G (I don't get that here in Australia, but I think it might be an option in other countries, I'm just guessing).
And... You'll learn to love 2.1 if you downgrade to 1.6 again, and find that your phone is performing horribly. :P